Heat-sensitive stencil paper

ABSTRACT

A heat-sensitive stencil paper for use in mimeographic printing, comprising a resinous film consisting of a vinylidene chloride-vinyl chloride copolymer which has a biaxial heat shrinking coefficient in the range of the order of 2-8% at 100 degree C. for 1 minute, said resinous film being lined with a porous thin sheet of paper, said stencil paper being so adapted that an image consisting of perforations in formed in said film by applying infrared light thereto.

United States Patent O fice 3,620,160 HEAT-SENSITIVE STENCIL PAPER Keishi Kubo and Kiyoshi Sakai, Tokyo, Japan, assignors to Kabushiki Kaisha Ricoh, Tokyo, Japan No Drawing. Filed Sept. 18, 1968, Ser. No. 760,718 Claims priority, application Japan, Oct. 6, 1967, 42/64,081 Int. Cl. B4ln 1/24 US. Cl. 101-1282 1 Claim ABSTRACT OF THE DISCLOSURE A heat-sensitive stencil paper for use in mimeographic printing, comprising a resinous film consisting of a vinylidene chloride-vinyl chloride copolymer which has a biaxial heat shrinking coeflicient in the range of the order of 2-8% at 100 C. for 1 minute, said resinous film being lined with a porous thin sheet of paper, said stencil paper being so adapted that an image consisting of perforations is formed in said film by applying infrared light thereto.

BACKGROUND OF THE INVENTION (a) Field of the invention The present invention is concerned with a heat-sensitive stencil paper for use in mimeographic printing, and more particularly, it relates to a heat-sensitive stencil paper which comprises a resinous film consisting of a vinylidene chloride-vinyl chloride copolymer which has a biaxial heat shrinking coefiicient in the range of the order of 28% at 100 C. for 1 minute and a porous thin sheet of paper, said resinous film being adapted so that an image consisting of perforations is formed in said film by applying infrared light thereto.

(b) Description of the prior art Heat-sensitive stencil papers which are of the type that a thin sheet of paper is adhesively bonded to a thermoplastic resinous film are known. A heat-sensitive stencil paper of this type is used in such a way that the stencil paper is exposed, through an image-carrying original laid thereon, to infrared light at the side of said thin sheet of paper, in which case the portions of the film corresponding to the image region in the original are subjected to the heat which has been accumulated, by absorption, in said image region of the original and, as a result, said portions of the film are melted in such a way that they are severed substantially in the middle part throughout the widths of said molten portions and that the regions of the molten resinous film located on both sides of the severance draw up and displace their positions in the directions away from their severad edges, resulting in that the portions of said thin sheet of paper lying beneath such portions of the film are now exposed through the opening formed between the severed edges of the film. An image consisting of perforations, which is used for the purpose of printing, corresponding to the original image is thus formed in the resinous film. Depending on the type of the resin employed, however, there often occurred cases where the displacement of the molten regions of the resinous film was not elfected uniformly because the resin had a wide range of melting point, and Where, accordingly, the printing sheet obtained was poor in the resolution of the image produced. The exact mechanism of how 3,620,160 Patented Nov. 16, 1971 such perforations are formed in a heat-sensitive stencil paper is very hard to elucidate. It cannot be explained in such a simple way as stated above. From the foregoing brief statement on the manner the perforations are formed, however, it may be considered appropriate to employ those particular thermoplastic resins which have both a narrow range of melting point and an intensive heat-shrinkability. In practice, however, the employment of vinylidene chloride-vinyl chloride copolymers (for example, those disclosed in the US. Pat. No. 3,267,847) which have a very narrow range of melting point are recommended.

SUMMARY OF THE INVENTION It is, therefore, the object of the present invention to provide a heat-sensitive stencil paper which is capable of forming a printing image consisting of an aggregation of perforations and being superior in resolution, by the employment of such a resinous film that consists of a vinylidene chloride-vinyl chloride copolymer and that has been made to have an appropriate heat-shrinkability by subjecting said film to biaxial drawing.

The relationship between the heat-shrinkability of plastic films and the resolving ability of such films which is exhibited when they are used in stencil papers has not been clarified yet. For this reason, we made an extensive research to find the magnitude of the heat-shrinking coefficient which is required of vinylidene chloride-vinyl chloride copolymers when the latter are used as the material of the films of stencil papers. As a result, it has been found that copolymers having an exceedingly small heat-shrinking coefiicient and those having an exceedingly large heat-shrinking coeificient are both not satisfactory for such a purpose, and also that those having a heatshrinking coefficient in the range of 2.08.0% biaxially at C. for 1 minute are most satisfactory. The present invention is based on this discovery.

As an example of resinous film having an extremely small heat-shrinking coeflicient, there was prepared a vinylidene chloride film having a heat-shrinking coefiicient of about zero at 100 C. for 1 minute, by the use of a commercial vinylidene chloride film (trade name: Krewrap, a product of Kureha Chemical Industry Co. of Japan) which, in itself, had relatively large heat-shrinking coetficient. This film was dissolved in tetrahydrofuran, and this solution was poured onto a flat plate so as to flow thereover, and then dried. The film which was formed in this Way was laminated on a porous thin sheet of paper and a perforation test was conducted on the stencil paper thus prepared by subjecting this stencil paper to infrared light. The result was that hardly any perforation was produced. By intensifying the application of infrared light, it was observed that perforations formed to some extent. Perforations were considered to have been formed in this instance in such a manner that the film Was melted first and then the molten film was transferred to the original which was laid on the fifilm. As a result, the original was badly soiled by the molten resin which adhered to the face of the original. Not only that, but also the image consisting of perforations was so poorly formed that it was not used for practical purposes. As such, vinylidene chloride films which have no or a very small heat-shrinkability cannot be used in heat-sensitive stencil papers.

With respect to vinylidene chloride films having an extremely high heat-shrinkability, on the other hand, there was made the following observation by us. In case, for example, a Vinylidene chloride film having a heat-shrinking coefficient of 30% at 100 C. for 1 minute was used, the lines constituting the image consisting of perforations resulting from said high heat-shrinkability of the resin were presented in the form of very wide lines. Thus, this image represented a markedly poor resolution. Such being the result, films having an exceedingly large heatshrinking coefiicient can not be used for practical puroses. p According to what has been made clear by us, vinylidene chloride-vinyl chloride copolymer films which can produce satisfactory results when used for practical purposes must have a heat-shrinking coefiicient 1n the range of 28% biaxially. Vinylidene chloride-vinyl chloride copolymers which satisfy this condition desirably have a Vinylidene chloride/vinyl chloride polymerizing proportion in the range of 7030/955. Also, the total amount of the additives used, such as a plasticizer, desirably is 20% or smaller relative to the aforesaid copolymer employed. Description will hereunder be directed to the method of manufacturing films of this type. Before doing so, reference will be made to the porous thin sheet of paper which can be applied to the present invention. Specifically, there can be used any of the following papers, i.e. papers made of natural fibers such as mitsumata fibers and manila hemp fibers; synthetic fibers such as rayon fibers, acrylic fibers and nylon fibers; and mixtures thereof.

Hereunder will be described an example of the manufacture of film.

(1) Film which is formed by processing a commercially available film A commercial Vinylidene chloride-vinyl chloride copolymer film (trade name: Krewrap, a product of Kureha Chemical Industries, Ltd. The Vinylidene chloride/vinyl chloride molar proportion: 90/10) is manufactured in general by extruding the molten polymer through an annular slit of an extruder, inflating this extruded polymer by air at 20-50 C. (draw ratio: 3-5 times), and then cooled to convert to a crystalline article. The resulting film has a thickness in the order of 0.01-0.02 mm. This film is then passed through a set of rolls, and the resulting collapsed annular film is cut at both side edges to produce two sheets of film which are wound around two separate cores, respectively. In this instance, the films have been drawn biaxially by the inflation by air. However, these films have a heat-shrinking coefficient of -20% in general. Therefore, in order that these films are used in the present invention, they may not or may be cut into an appropriate size, and they are either treated with a solvent or given a heat treatment so that they will have a reduced heat-shrinking coefficient in the range of 28%.

(2) Methods of forming a film As the methods of forming a film with a thermoplastic resin, there are the extrusion process which utilizes a T- die; the casting process; the calendering process; the skiving process; the emulsion process; and some others. Many of the films obtained from these known processes, however, either lack heat shrinkability or they are oriented in only one direction. Therefore, these films require to be given some sort of drawing process so that they will have a heat shrinking coefiicient of the order of 28% biaxially in order that the films may be made applicable to the present invention. Among the films formed by these known film-forming processes, those obtained by the casting process and those by the extrusion process utilizing a T- die, of all other processes, are adaptable to the present invention. In order to effect their drawing, the employment of the tenter process is appropriate. The films ob- 4 tained by the inflation process can be applied to the present invention provided that the expansion of the film by air inflation is low in degree.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1 To a synthetic resin powder made of a copolymer consisting of of vinylidene chloride and 15% of vinyl chloride were added 4% by weight of 3-(2-xenyloxy)-1,2- epoxypropane and 4% by weight of 4-tertiary butylphenyl salicylate, and thus, a composition was prepared. This composition was then subjected to molding by the air inflation technique to produce a film having a transverse heat-shrinking coefficient (at C., for 1 minute) of 12% and a longitudinal heat-shrinking coefficient of 20% and a thickness of 810/.L. Thereafter, thisfilm was immersed for 10 seconds in acetone which was held at about 20 C., and then dried in the air at 20 C. The resulting film showed a transverse heat shrinkability of 4% and a longitudinal heat shrinkability of 7%. This film was then bonded to a porous thin sheet of paper having a thickness of 40 As a result, a satisfactory heat-sensitive stencil paper was obtained.

An original carrying printed images on both sides was laid on this stencil paper and they were subjected to the so-called sliding exposure by sliding this combination below an infrared ray lamp at a speed of 40 mm./sec. to form perforations in the film, and thus a perforated printing sheet for use in mimeographic printing was prepared.

Using this printing sheet, printing was performed by the use of an ordinary rotary stencil press. The result was that a print which faithfully reproduced the image of the original and which carried a clear printed image was obtained. At the end of more than 2000 prints that were produced, there was noted no loss of clarity in the printed images.

Example 2 A similar experiment was conducted by the use of methylethylketone (MEK) instead of the aforesaid acetone, but the result was substantially the same as that obtained in the preceding example.

Example 3 A 10% tetrahydrofuran solution of a resin made of a copolymer consisting of 88% by weight of Vinylidene chloride and 12% by weight of vinyl chloride was prepared. A small amount (about 3% by weight) of di-(2- phenylethyl) ether and 2-hydroxy-5-chlorobenzophenone was added to said solution. By the use of an endless belt made of nickel and having a well polished surface coated with a film of polyvinyl alcohol, the aforesaid mixed solution was subjected to casting process at the film-forming speed of 10 m/min. and thus, a film of 20;/. in thickness was obtained. While heating this film, the latter was drawn longitudinally to a length twice the original length. Thereafter, the resulting film was led to a tenter wherein it was drawn transversely twice the original length. While holding the bilateral edges of the film in the rearward section of the tenter, the film was heat set. As a result, the film was converted to homogeneous one which had a heat shrinking coefiicient of about 3% at 100 C. for 1 minute biaxially. A perforated printing sheet was prepared with this film in a manner similar to that described in Example 1, and the printing sheet was tested. The result was from comparable to superior to the result obtained in Example 1.

It is added that, in the foregoing examples, the measurement of the heat-shrinking coefficient was conducted in accordance with ASTM D1204-54.

What is claimed is: References Cited 1. In a heat-sensitive stencil paper comprising a synthetic resin film bonded to a porous thin fibrous sheet, the UNITED STATES PATENTS 2,699,113 1/1955 Hoover 101128.4

principal component of said film being vinyliden'e chloride-'vinyl chloride copolymer, the improvement in which 5 3,267,847 8/1966 Hayama et a1 101 128-2 DAVID KLEIN, Primary Examiner the mol ratio of vinylidene chloride/vinyl chloride in said film is in the range of 70/ 30-95/ 5 and the film contains at least 80% by weight of said vinylidene chloride-vinyl U S C1 X R chloride copolymer, said film having a biaxial heat-shrinking coetficient in the range of 2% to 8% measured at 10 100 C. for one minute. 

